Improving the delivery of drugs and other agents to target tissues has been the focus of considerable research for many years. Most agents currently administered to a patient parenterally are not targeted, resulting in systemic delivery of the agent to cells and tissues of the body where it is unnecessary, and often undesirable. This may result in adverse drug side effects, and often limits the dose of a drug (e.g., cytotoxic agents and other anti-cancer or anti-viral drugs) that can be administered. By comparison, although oral administration of drugs is generally recognized as a convenient and economical method of administration, oral administration can result in either (a) uptake of the drug through the epithelial barrier, resulting in undesirable systemic distribution, or (b) temporary residence of the drug within the gastrointestinal tract. Accordingly, a major goal has been to develop methods for specifically targeting agents to cells and tissues that may benefit from the treatment, and to avoid the general physiological effects of inappropriate delivery of such agents to other cells and tissues.
In addressing this issue, some investigators have attempted to use chimeric molecules that bind to growth factor receptors on gastrointestinal epithelial cells to facilitate transepithelial transport of therapeutic agents (see WO 93/20834). However, these methods have several disadvantages. For example, such chimeric molecules are transcytosed through the epithelium from the gut lumen and absorbed into the blood stream, resulting in systemic distribution and removal from the epithelium proper. Since the therapeutic agents are targeted specifically away from the epithelium for systemic distribution, these chimeric molecules are generally not useful for treatment of epithelium associated conditions. In addition, TGF-α or other molecules binding to EGF receptors exhibit many or all of the apparent biological activities of EGF, such as stimulation of enterocyte mitogenesis or suppression of gastric secretion. Such effects collateral to the transcytotic uptake of therapeutic agents may not be desirable or may be contraindicated for intervention of epithelium associated conditions or diseases. Furthermore, EGF receptors are not unique to epithelial cells of the gastrointestinal tract, and can be found on numerous other cells including kidney cells and hepatocytes. Thus, molecules which have affinity for the EGF receptor and are distributed systemically in the blood can be rapidly removed from circulation, accumulated in specific organs and potentially degraded or secreted.
Within an alternative approach, other investigators have employed Fab fragments of an anti-polymeric immunoglobulin receptor IgG to target DNA to epithelial cells in vitro that contain such a receptor (see Ferkol et al., J. Clin. Invest. 92:2394-2400, 1993). Still other researchers have described the translocation of a chimeric IgA construct across a monolayer of epithelial cells in vitro (see Terskikh et al., Mol. Immunol. 31:1313-1319, 1994). Others have used ascites tumor implants in vivo in mice and observed an IgA dimeric antibody produced by subcutaneous tumor cells to accumulate in feces, suggesting that IgA is transported across an epithelial barrier of the gastrointestinal tract (see Greenberg et al., Science 272:104-107, 1996).
Notwithstanding the above-noted developments, there remains a need in the art for systems for delivering agents to target cells, particularly epithelial cells and cells or tissues bounded by epithelial cells. The present invention fulfills these needs and further provides other related advantages.